The present invention relates to turbine systems. More particularly, the invention relates to components of such turbine systems. Still more particularly, the invention relates to turbine components formed from a molybdenum silicide-based composite. Finally, the invention relates to environmentally resistant coatings for such turbine components.
Turbine systems, such as, but not limited to, aircraft turbines, land-based, turbines, marine-based turbines, and the like, and their components (hereinafter referred to as “turbine components”) have typically been formed from nickel (Ni) based materials, which are often referred to as Ni-based superalloys. Turbine components formed from these Ni-based superalloys exhibit desirable chemical and physical properties under the high temperature, high stress, and high-pressure conditions generally encountered during turbine operation. The highest surface temperatures of state-of-the-art jet engine turbine airfoils, for example, reach as high as about 2100° F. (about 1150° C.), or about 85% of the melting temperature (Tm) of most of the Ni-based superalloys.
To date, the Ni-based superalloys have provided the desired level of performance for turbine system applications, causing the development of such Ni-based superalloys to be widely explored. As a result of such extensive study, the field has matured and few significant improvements have been realized in this area in recent years. In the meantime, efforts have been made to develop alternative turbine component materials.
These alternative materials include composite materials (hereinafter generally referred to as “molybdenum silicide-based composites”) that are based on molybdenum silicide, such as MoSi2, Mo3Si, Mo5Si3, and Mo5SiB2 (T2) Most molybdenum silicide-based composites have melting temperatures of greater than about 3100° F. (about 1700° C.). If molybdenum silicide-based composites can be used at about 80% of their melting temperatures, they will have potential use in applications in which the temperature exceeds the current service temperature limit of Ni-based superalloys.
Molybdenum silicide-based composites comprising molybdenum (Mo), silicon (Si), and either boron (B) or chromium (Cr) are among the materials that have been proposed for turbine component applications in which Ni-based superalloys are presently used. These molybdenum silicide-based composites exhibit a high temperature capability which exceeds that of the Ni-based superalloys that are currently used in such applications. Exemplary molybdenum silicide-based composites are described by Berczik (U.S. Pat. Nos. 5,595,616 and 5,693,156), and Subramanian et al. (U.S. Pat. Nos. 5,505,793 and 5,683,524).
Although the molybdenum silicide-based composites show potential for use as next-generation turbine components having service temperatures that are significantly greater than those of current Ni-based superalloy components, the oxidation of such turbine components remains a concern. At temperatures in the range between about 2000° F. and about 2600° F. (between about 1090° C. and about 1425° C.), refractory materials can undergo rapid oxidation. While a slow-growing oxide scale can form on molybdenum silicide-based composites at this temperature, it is typically not a protective oxide scale. Another type of oxidation known as ‘pesting’ occurs at intermediate temperatures (e.g., between about 1000° F. and about 1800° F.). Pesting is a phenomenon that is characterized by the disintegration of a material into pieces or powders after exposure to air at intermediate temperatures. Refractory metals, particularly molybdenum, exhibit poor resistance to pesting oxidation.
While significant progress has been made in improving the oxidation performance of molybdenum silicide-based composites, it is desirable to provide coatings for turbine components fabricated from these materials in order to ensure long lifetimes at service temperatures of 2000° F. to 2600° F. Therefore, what is needed is a turbine system that includes components formed from molybdenum silicide-based composites, the components having coatings that will provide increased oxidation resistance at temperatures in the range between about 2000° F. and about 2600° F. What is further needed is a turbine system comprising molybdenum silicide-based composite components having coatings that will provide increased resistance to pesting at temperatures between about 1000° F. an about 1800° F. What is also needed is an environmentally resistant coating for molybdenum silicide-based composites, which will enhance oxidation resistance at high temperatures and pesting resistance at intermediate temperatures.